1. Field of the Invention
This invention relates to a Faraday rotation glass having a large Verdet constant and a small nonlinear optical constant.
2. Description of the Prior Art
The rotation of the plane of polarization of a beam of linearly polarized light when the light passes through a transparent material in a magnetic field is known as the Faraday effect. The angle .theta. of the rotation is given by the following equation EQU .theta.=VHl
where .theta. is the angle of rotation measured in minutes, H is the strength of the magnetic field in gausses, l is the length of the light path in the material through which the light passes in centimeters, and V is the Verdet constant inherent to the substance.
This phenomenon is utilized, for example, in a light isolator which permits the transmission of light only in one direction. In this case, it is important for the Verdet constant of the material to be large. It is known that a glass containing large quantities of paramagnetic rare earth element oxides has a large Verdet constant, and in particular, since terbium oxide (Tb.sub.2 O.sub.3) does not have an inherent absorption in the visible and infrared regions, terbium oxide is an effective ingredient of a Faraday rotation glass.
When a Faraday rotation glass is used as a light isolator element in a large-output nuclear fusion laser optical system, the glass needs to have a low nonlinear optical constant n.sub.2 (namely, a low refractive index) in addition to having a large Verdet constant. This is because a glass with a lower n.sub.2 causes lesser self-focusing of laser beams, and therefore is less subject to damage by the self-focusing effect.
In a glass containing Tb.sup.3+ ion, the Verdet constant increases as the content of Tb.sup.3+ ion increases. At the same time, the refractive index of the glass increases and so does n.sub.2. Hence, in order to obtain a glass having a large Verdet constant and a small nonlinear optical constant n.sub.2, a glass having a low refractive index which can contain a large quantity of a Tb.sup.3+ ion must be used. From this standpoint, Tb-phosphate glasses are preferred to Tb-borosilicate glasses.